1. Field of the Invention
The present invention relates to a thermoplastic resin film and a process for producing the same, more particularly to a thermoplastic resin film having a small variation in thickness and/or having a high elastic modulus and a high F-5 value, and a process for producing the same.
2. Description of the Prior Art
In production of a thermoplastic resin film, uniformity of thickness is an important basic quality. For example, a polyester film, which is a typical thermoplastic resin film, is employed for various uses such as a base film for magnetic recording media, an insulating material for capacitors and other electrical elements and devices, and a material for office automation equipment such as printers because of excellent properties thereof. In these uses, high uniformity in thickness and excellent mechanical properties are required. Therefore, minimizing the variation in thickness of a film and providing toughness to the film are very important.
As causes generating variations in thickness, there are various factors such as variations in the output of a molten resin delivered from a die when the molten resin is cast on a cooling drum in a form of a sheet or film, vibration of the resin film between the die and the cooling drum and variations in in the rotational speed of the cooling drum, and so on. When a biaxially stretched film is formed, besides these factors, there are other factors such as non-uniformity in surface temperature, variation in rotational speed of rollers employed for longitudinal stretching, non-uniformity in temperature and variation in wind speed of hot air in a tenter employed for transverse stretching.
Accordingly, various methods for limiting thickness variation in a film have been proposed. For example, a method for suppressing a variation of the rotational speed of a casting drum cooling and solidifying a molten resin is disclosed in JP-A-SHO 55-93420 and a method for modifying a resin such that the resin can be easily treated with an electrostatic force when a molten resin is brought into contact with a casting drum by the electrostatic force is disclosed in JP-A-SHO 59-91121. However, the results of these methods are not sufficient.
Further, a method for extruding a resin at a temperature not higher than the melting point of the resin is considered to be effective in order to prevent vibration in a film of a molten resin. For example,methods for extruding a resin at a temperature of not higher than the melting point of the resin are disclosed in JP-B-SHO 53-11980, JP-B-SHO 53-19625 and JP-B-HEI 1-55087. In these methods, however, a resin is cooled at a temperature of not higher than the melting point of the resin at a position upstream of a land of a die and when the resin after the cooling is supplied to the land portion of the die, the resin is molecularly oriented therein by reducing the cross section. In addition, after cooling the resin at a temperature of not higher than the melting point within the area inside of the die and almost solidifying the resin, the resin is supplied to the land, and the resin is extruded from the die while a shear stress is applied to the resin in the land portion. Therefore, in these methods, an extremely high pressure for extrusion is required, it is difficult to extrude the resin by a usual extruder and a particular extruder for high-pressure extrusion is required, it has a poor extrusion stability and only a film having a great variation in thickness is obtained. Further, a load applied to a die body or the whole of the die is large, thereby causing a deformation thereof and a decrease of durability thereof. Furthermore, because it is extremely difficult to extend the almost solidified resin in the width direction of the die, even if it can be extended, a film obtained becomes poor in uniformity in thickness originating from a variation of the resin flow, and the advantage aimed by the present invention to suppress a variation in thickness cannot be expected.
Further, in order to obtain a film having a high elastic modulus, generally it is required to further stretch a biaxially stretched film in a required direction. Therefore, in such a film, the yield in production is low and the cost for production becomes high, also a film having many surface defects, poor thermal dimensional stability and great variation in thickness is obtained.
Thus, although various methods have been proposed for improving a variation in thickness of a film, the effects thereof are still insufficient. Further, a film having a high elastic modulus, a good uniformity in thickness, a good thermal dimensional stability and no surface defects at the same time has not been obtained.